Method and network node for selecting a combining point

ABSTRACT

The present invention relates to a method and network device for selecting a combining point at which at least two redundant transmission paths are combined to a single transmission path in a transmission network comprising at least two selectable combining points (B-J). The combining point is selected by a method using at least two measurement-based selection criteria to which different priorities are allocated. The selection result of a selection criterion with a higher priority is used as a constraint for a selection based on a selection criterion with a lower priority. The selection criteria are applied to lengths or loads of the redundant transmission paths or the single transmission path. Thereby, an optimized combining point can be obtained to thereby lower delays for combined traffic and increase efficiency of network utilization.

FIELD OF THE INVENTION

The present invention relates to a method and network node for selectinga combining point, e.g. a Macro Diversity Combining (MDC) point, atwhich at least two redundant transmission paths are combined to a singletransmission path in a transmission network, such as a radio accessnetwork (RAN) providing access to an Internet Protocol (IP) basednetwork architecture, comprising at least two selectable combiningpoints.

BACKGROUND OF THE INVENTION

In a Code Division Multiple Access (CDMA) based cellular network allusers in the same cell or in different cells may share the samefrequency spectrum simultaneously. In spread spectrum transmission, theinterference tolerance enables universal frequency reuse. This enablesnew functions such as soft handover, but also causes strict requirementson power control. Due to the universal frequency reuse, the connectionof a radio terminal, e.g. a mobile terminal, mobile station or userequipment to the cellular network can include several radio links. Whenthe radio terminal is connected through more than one radio link, it issaid to be in soft handover. If, in particular, the radio terminal hasmore than one radio link to two cells on the same side, it is in softerhandover. Soft handover is a form of diversity, increasing thesignal-to-noise ratio when the transmission power is constant. Atnetwork level, soft handover smoothes the movement of a mobile terminalfrom one cell to another. It helps to minimize the transmission powerneeded in both uplink and downlink.

Thus, a radio terminal of a network subscriber can transmit the sameinformation on a plurality of redundant transmission parts that are setup parallel via a radio transmission interface from the cellular networkto the radio terminal or from the radio terminal to the cellular networkin order to achieve an optimal transmission quality. Such a transmissionstructure is called macro diversity. The redundant transmission pathscan be dynamically setup and cleared down while the radio terminalchanges its location. The information sent out by the radio terminal inthe transmission frames on various transmission paths can be merged inthe transmission network at combination points at which respectively twotransmission paths are combined into a single transmission path in onetransmission direction (uplink) and the single transmission path isdivided into two transmission paths in the other transmission direction(downlink). A corresponding network architecture is described forexample in the U.S. Pat. No. 6,198,737 B1.

In order to obtain the most efficient RAN architecture, which is basedon using advantageous characteristics of IP, some functionality isrelocated between network elements. According to a recent new RANarchitecture, a network element known as Base Station Controller (BSC)or Radio Network Controller (RNC) is no longer required, although thisfunctionality must remain in the RAN architecture. Therefore, thelocation of a combining point, e.g. MDC point, can no longer becentralized for all base stations in the RAN. Consequently, some RNCfunctionality has been transferred to the base stations in order toenable soft handover and associated signaling to happen along theshortest path, producing minimum delay and signaling load to those pathsof the network where this is not necessary. This new RAN architecture isdescribed e.g. in the White Paper “IP-RAN, IP—the future of mobility”,Nokia Networks, 2000.

In such a new RAN architecture, the MDC point can be selecteddynamically e.g. by a serving base station instead of having thisfunctionality in one pre-selected point like the RNC in the conventionalRAN architecture or in the base station that initiates the call. In thenew RAN architecture, base stations are able to act as MDC points.However, it should be possible to limit this set in order to reduce thenumber of MDC point relocations, which introduce additional delay, i.e.,only some base stations can act as MDC points if needed. Those basestations are called MDC-capable base stations or simply MDC-capableBTSs.

However, if the first common upstream base station, i.e. the basestation closest to the radio network gateway on the common path from aserving base station towards any drift base station, is always selectedas the MDC point for the base stations that participate in softhandover, the processing load of the MDC point might be too high andnetwork resources are not optimized. Moreover, it might be desired toperform link load balancing by selecting a more appropriate base stationas the MDC point.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand network node for selecting a combining point in a transmissionnetwork, by means of which the load at the combining point can bereduced and a more efficient network utilization can be achieved.

This object is achieved by a method of selecting a combining point atwhich at least two redundant transmission paths are combined to a singletransmission path in a transmission network comprising at least twoselectable combining points, said method comprising the steps of:

-   using at least two measurement-based selection criteria for    selecting said combining point;-   allocating different priorities to said at least two selection    criteria; and-   using the selection result of a selection criterion with a higher    priority as a constraint for a selection based on a selection    criterion with a lower priority.

Furthermore, the above object is achieved by a network node forselecting a combining point at which at least two redundant transmissionpaths are combined to a single transmission path in a transmissionnetwork comprising at least two selectable combining points, saidnetwork node being arranged to use at least two measurement-basedselection criteria with different priorities for selecting saidcombining point, and to use the selection result of a higher priorityselection criterion as a constraint for a selection based on a lowerpriority selection criterion.

Accordingly, the combining point location is optimized based on a goalfunctionality, e.g. a preemptive method, which works in such a fashionthat an optimal solution to a highest priority goal is searched and thissolution is added as a new constraint for lower priority goals. If thesolution for a higher priority goal leads to a single combining point,lower priority goals may not have to be considered. Such a preemptivemethod is advantageous in that it always results in an optimum value forthe highest priority goal. Moreover, only a decision regarding thepriority order of the available different goals is required, while noweights have to be determined. Thereby, lower delays for MDC traffic anda more efficient network utilization can be obtained due to theoptimized location of the combining point.

Preferably, the at least two selection criteria comprise a selectioncriterion applied to measured lengths or loads of the at least tworedundant transmission paths and/or the single transmission path.Furthermore, the at least two selection criteria may comprise aselection criterion applied to measured processing loads of theselectable combining points. In particular, the at least two selectioncriteria may comprise a first criterion of minimizing the maximum lengthof the at least two redundant transmission paths, a second criterion ofminimizing the maximum total length of the at least two redundanttransmission paths and the single transmission path, a third criterionof minimizing the maximum traffic load on the at least two redundanttransmission paths and the single transmission path, and a fourthcriterion of minimizing the processing load of the combining point. Themaximum length and maximum total length may be determined by countinghops of said single and redundant transmission paths, respectively.Furthermore, the highest priority may be allocated to the firstcriterion, the second highest priority to the second criterion, thethird highest priority to the third criterion, and the lowest priorityto the fourth criterion. The third criterion may be applied bymonitoring and updating real time traffic loads using an averagingfunction, e.g. an exponential averaging function.

Thus, the role of load measurements, used for measuring both link loadsand combining point processing loads, is emphasized in selecting theoptimal combining point. This provides the advantage that both linkloads and combining processing loads can be balanced by selecting theoptimal combining point.

The load measurements results may be transmitted mutually between the atleast two selectable combining points at predetermined intervals.Alternatively, the load measurement results or load reports may betransmitted from the at least two selectable combining points to acentralized resource, which will distribute the load information to allpossible combining points at predetermined intervals. As a furtheralternative, the load reports may be transmitted from the at least twoselectable combining points directly to all other possible combiningpoints at predetermined intervals without any intervention of acentralized resource. In this connection, load measurement resultsrefers to “raw data” from router statistics while load report containsalready processed information.

Furthermore, maximum load thresholds may be set to be considered duringthe selection of the combining point. The maximum real time loadthreshold may define the maximum allowable real time load on used linksof the at least two redundant transmission paths and other (class x)load thresholds may define the maximum allowable load for differenttraffic types (e.g., streaming) on the single transmission path.Furthermore, a maximum load threshold may be provided for defining themaximum allowable processing load in the selected combining point.

A selection criterion may be bypassed if the required measurement valuesare not available.

If the selection method does not lead to a selection of a combiningpoint, a redundant transmission path may be dropped from the at leasttwo redundant transmission paths. Finally, if only one redundanttransmission path is left and the method still does not lead to aselection of a combining point, the corresponding call may invoke one ofsome not yet mandated actions. For example, the possible actions may be(1) using the remaining base station in the active set of the on-goingcall as the new serving BTS or (2) keeping the current MDC point of theon-going call unchanged or (3) rejecting the new call (new call meansthat connection hasn't been set up yet) as a whole.

The selection method may be used after a change of the network topology.

The combining point may be an MDC point, if the selection method is usedin a universal radio access network for providing access to an IP-basednetwork. In this case, the selectable combining points may be basestation devices. The network node for performing the selection methodmay be e.g. a base station device or centralized resource managingdevice.

Additionally, a combining point validity checking functionality may beapplied, wherein a previously selected combining point is maintained ifthe previously selected combining point at least still meets the atleast two measurement-based selection criteria. The checkingfunctionality can be modified such that at least one stricter selectioncriterion is applied to the previously selected combining point. As anexample, the at least one stricter selection criterion may correspond to90% of a load threshold value applied to the previously selectedcombining point.

Furthermore, a fallback scheme of MDC selection on topology informationinconsistency may be provided at or for an MDC module, wherein atopology inconsistency is detected, and relocations of the combiningpoint are prevented during the detected topology inconsistency. In thiscase, e.g. a timer function may be started in response to the detection,and relocations are then allowed again after the expiry of the timerfunction.

A subset of nodes capable of being selected as the combining point inthe selection step may be determined e.g. based on the topology of saidtransmission network. This determination may be repeated after a networktopology change. The subset of capable nodes may be selected based ontheir number of links connecting to other nodes, e.g., those nodeshaving a predetermined number of links, e.g. two links, or more areselected.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on a preferred embodiment with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic diagram of a radio access network topology inwhich the present invention can be implemented;

FIG. 2 shows a flow diagram indicating a selection method according tothe preferred embodiment of the present invention;

FIG. 3 shows an algorithm for a first selection criterion according tothe preferred embodiment;

FIG. 4 shows an algorithm for a second selection criterion according tothe preferred embodiment;

FIG. 5 shows an algorithm for a third selection criterion according tothe preferred embodiment;

FIG. 6 shows an algorithm for a fourth selection criterion according tothe preferred embodiment;

FIG. 7 shows a table indicating combining processing loads and real timetraffic loads according to a specific implementation example;

FIG. 8 shows an algorithm for a validity checking function;

FIG. 9 shows an enhancement of the flow diagram in FIG. 2 to incorporatethe validity checking function;

FIG. 10 shows a specific example for the algorithm of FIG. 9;

FIG. 11 shows an algorithm for detecting a topology informationinconsistency;

FIG. 12 shows an algorithm for a topology information inconsistencyfallback sub-scheme;

FIG. 13 shows an example of a first RAN topology with a topologyaggregation scheme; and

FIG. 14 shows an example of a second RAN topology with topologyaggregation scheme.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment will now be described on the basis of a new RANnetwork architecture for providing access to an IP network.

According to FIG. 1, a mobile terminal M is connected to a RAN via threeredundant transmission paths indicated by respective dash-dot lines. TheRAN architecture comprises a plurality of network nodes A to J, whereinthe shaded nodes E, G and H are currently connected to the mobileterminal M via the redundant transmission paths. In particular, thenetwork node H indicated with the bold circle is used as the servingbase station, i.e. the base station terminating the core networkinterfaces data stream, and performing Radio Resource Management (RRM)functions like scheduling, power control and the like. In contrastthereto, the other shaded base stations G and E are used as drift basestations providing only resources and radio L1 layer functions for therespective connections to the mobile terminal M.

In the RAN topology shown in FIG. 1, contrary to conventional RANs, mostof the functions of the former centralize controller (RNC or BSC) aremoved to the base stations. In particular, all radio interface protocolsare terminated in the base stations. Entities outside the base stationsare needed to perform common configuration and some radio resourcefunctions, or interworking with legacy, gateways to a core network, etc.An interface is needed between the base stations, supporting bothcontrol plane signaling and user plane traffic. Full connectivity amongthe entities may be supported over an IPv6 (Internet Protocol version 6)transport network. The network node A indicated by a double circularline corresponds to a RAN Gateway (RNGW), which is the IP user planeaccess point from the IP-based core network or other RAN to the presentRAN. During a radio access bearer assignment method, the RAN returns tothe core network transport addresses owned by the RNGW A where the userplane shall be terminated. Additionally, packet-switched andcircuit-switched interfaces are connected through the RNGW A. The mainfunction of the RNGW A is a micro-mobility anchor function, i.e. theuser plane switching during the relocation/handover of base stations, inorder to hide the mobility to the IP-based core network. Due to thisfunction, the RNGW A does not need to perform any radio network layerprocessing on the user data, but relays data between the RAN and IPtunnels of the IP-based core network. Thus, the RNGW A is responsiblefor the association between RAN tunnels and core network tunnels, setupand release of tunnel endpoints, user plane traffic switching, packetrelaying, mapping between tunnel endpoint IDs, and firewall/securityfunctions. It is noted that several RNGWs may be provided in the RAN toguarantee a flexible relationship between the RNGWs and the basestations.

In the situation shown in FIG. 1, the network node F has been selectedas the MDC point for the connections to the mobile terminal M. Thedashed arrows indicate the MDC legs between the serving and drift basestations H, G and E and the MDC point F. Furthermore, the dotted arrowbetween the MDC point F and the RNGW A indicates the single transmissionpath to which the redundant transmission paths are combined. Thus, theterm “MDC leg” refers to the path from the MDC point F to one of thedrift and serving base stations H, G and E.

According to the preferred embodiment, a measurement-based method isprovided for selecting the most appropriate MDC point among the networknodes, e.g. base stations B to J in the IP-based RAN. To achieve this,the MDC point location is optimized using a goal programming e.g. byfirst minimizing the maximum MDC leg length, then minimizing the totalhop count of MDC legs and the paths from the RNGW A to the MDC point,then minimizing the maximum real time traffic load on used links, andfinally minimizing the MDC processing load at the potential MDC point.

It is noted that the term “real time traffic” refers to packets markedwith certain differentiated services code points, e.g. ExpeditedForwarding (EF) which indicates a high priority Per-Hop Behavior (PHB)to achieve a high-quality virtual circuit service. The term “hop” isused to denote a path between two network nodes, that does not have anysignificant effect on the characteristics of traffic flows. Thus, aconnection path between two neighbor network nodes in FIG. 1 correspondsto one hop. The PHB is an externally observable forwarding treatment ofan aggregate traffic stream in a network node.

Real time traffic loads on all links are monitored and updated by usingexponential averaging according to the following equation:rt_load_(i)=(1−w)*rt_load_(i-1) +w*(rt_bits/(p*link_(—) bw)),  (1)wherein rt_bits denotes the number of real time bits sent on an outputlink, link_bw denotes the output link bandwidth, which may be bothobtained from router statistics, w denotes an exponential averagingweight, and p denotes a measurement period. An appropriate value for pcould be e.g. 500 ms. The value for w depends on the desired reactiontime to changes in link loads, e.g. w=0.5. A similar averaging mechanismmay be applied to all other relevant link loads.

Furthermore, the ratio of on-going MDC connections and the maximumnumber of MDC connections could be monitored for every base station. Thecorresponding threshold values may vary considerably between differentbase stations. E.g., star points should be able to handle more MDCconnections than a base station at the end of a chain. Here,instantaneous values could be used instead of the above exponentialaveraging.

The number of MDC connections running in a particular base station andtraffic load information on all links directly attached to thisparticular base station may be transmitted periodically, e.g. every pms, as measurement results to other base stations. To avoid excessivetraffic, a multicasting-like approach (e.g., Spanning Tree basedalgorithm) could be used. Furthermore, the value of the parameter pcould be increased if the number of base stations is increased in theRAN architecture.

Alternatively, if a centralized IP Transport Resource Manager (ITRM) orbandwidth broker is provided, each base station could send itsmeasurement results to the ITRM only. The ITRM (not shown in FIG. 1)would then use a multicast-like approach to periodically distribute“aggregate” load reports to all MDC-capable base stations If acentralized resource is not used for the load reports distribution, anyBTS in RAN then sends its load report directly to all other MDC-capableBTSs at predetermined intervals, where a multicasting-like approach maybe used for the sending. In any way, the messages containing themeasurement results or load reports should be given a high priority interms of packet delay and loss, e.g. by marking them with an appropriatedifferentiated services code point (e.g. EF).

Furthermore, it is assumed that all network nodes of the RAN networkhave a “helicopter view” of the network topology. This can be achieved,for example by using OSPF (Open Shortest Path First) routing protocol,as described in John T. Moy, “OSPF: Anatomy of an Internet RoutingProtocol, 3^(rd) printing, September 1998, ISBN 0-201-63472-4. In thisrespect, it will be noted that only a single (shortest) path is in usebetween two network nodes if basic OSPF is used. Nevertheless, loadbalancing can be performed by choosing an appropriate MDC point.

The following priority order may be given to the above goals orselection criteria for selecting the appropriate MDC point. The highestpriority may be allocated to the goal of minimizing the maximum MDC leglength. The second highest priority may be allocated to the goal ofminimizing the total hop count of MDC legs and the paths from the RNGW Ato the MDC point. The third highest priority may be allocated to thegoal of minimizing the maximum real time traffic load on used links.Finally, the lowest priority may be allocated to the goal of minimizingof the MDC processing load, e.g. the number of on-going connectionsdivided by some predetermined threshold value.

Additionally, at least one of the following general constraints may beset. The maximum real time traffic load on MDC legs (max_rt_load) shouldbe less or equal than a real time threshold value (rt_threshold).Maximum class x (traffic class of packets sent from RNGW; can be realtime as well) traffic load on the path between RNGW and MDC pointcandidate (max_x_load) should be less or equal than a class x thresholdvalue (x_threshold). Furthermore, the MDC processing load in the MDCpoint candidate (mdc_load) should be less than MDC threshold value(mdc_threshold).

FIG. 2 shows a flow diagram indicating an implementation of theselection processing according to the preferred embodiment, in which theabove goals and parameters are used. Furthermore, in the following therunning variable N denotes the number of MDC point candidates, e.g. basestations in the RAN architecture, and the running variable M denotes thenumber of drift base stations participating in a particular softhandover situation.

The separate flow diagram on the upper right portion of FIG. 2 indicatesa method for distributing individual measurement results obtained at thebase stations either to the other base stations in the RAN or to thecentral ITRM node (not shown in FIG. 2), after an exponential averagingwith a weight parameter w has been applied to the measured link loads,and a timer has expired. Furthermore, another separate flow diagram isindicated below the above described separate diagram, wherein this lowerflow diagram indicates a processing for updating the MDC processing loadin the MDC point candidates whenever a new MDC connection starts or aMDC connection is dropped. These two partial or sub-methods arecontinuously performed in the background or in parallel to the followingmain selection method starting at the upper left side of FIG. 2.

When a request for a new call or some other trigger arrives at a servingbase station (IP BTS), the first selection criterion is applied in step1, wherein the maximum MDC length is minimized. Then, the selectionresult is compared to the general constraints, e.g. real time threshold,class x threshold and/or MDC threshold. If these constraints are notmet, it is checked whether multiple base stations are left. If so, thebase station with the worst radio connection to mobile phone is droppedand the selection method based on the first criterion with the highestpriority is repeated. If a single base station is left and theconstraints are still not met, the call is rejected—if it is a new call.In the case of an on-going call, we continue the call without softhandover (keeping its current MDC point unchanged or with the remainingBTS acting as serving BTS).

If general constraints are met, it is checked whether a single MDC pointhas been obtained by the first selection operation. If so, this singleMDC point is output as the best or appropriate MDC point. If not, thesecond selection criterion with the second highest priority is appliedin step 2 using the additional constraint or result of the selection instep 1.

FIG. 3 shows an example for an algorithm corresponding to the highestpriority selection criterion in step 1. Initially, minimum and maximumMDC hop counts are set, e.g. 100. Then, it is checked for every MDCpoint candidate, whether the MDC threshold, class x threshold and thereal time threshold constraints are met. If so, the maximum hop count isset to the hop count of the path between the candidate and the servingbase station, i.e. the length of the redundant transmission path. Then,it is checked for all drift base stations, whether the hop count betweenthe candidate and the drift base station is larger than the set maximumhop count. If so, the maximum hop count is set to the hop count betweenthe candidate and the drift base station. If the resulting maximum hopcount is smaller or equal than the set minimum hop count, the minimumhop count is set to the obtained maximum hop count. If the obtainedmaximum hop count is larger than the set minimum hop count, thecandidate is dropped from the MDC point candidate list. Furthermore, ifthe initial threshold constraints are not met for the actual candidate,it is also dropped from the MDC point candidate list.

In step 2 of FIG. 2, the number of total hops of the MDC candidatesobtained from the first criterion in step 1 is minimized. If a singleMDC point is obtained as the result of the second criterion, it isoutput as the best or appropriate MDC point.

FIG. 4 shows an algorithm as an example of the selection methodaccording to the second criterion with the second highest priority.Initially, a minimum hop count is set to a predetermined value, e.g.100. Then, a total hop count of all paths between the candidate and theserving and drift base stations and between the RNGW A and the candidateis determined for each remaining candidate and compared to the minimumhop count. If the obtained hop count of a candidate is smaller or equalthan the minimum hop count, the minimum hop count is set to the hopcount of the candidate. If not, the candidate is dropped from the listof remaining MDC point candidates.

In step 3 of FIG. 2, the third criterion with the third highest priorityis applied by minimizing the maximum real time traffic load for the MDCpoint candidates obtained from step 2. If the result of step 3 leads toa single MDC point, this single MDC point is output as the best orappropriate MDC point. If not, the fourth criterion with the lowestpriority is applied in step 4.

FIG. 5 shows an example of an algorithm, which may be used in step 3.Initially, a minimum value for the maximum load is set to apredetermined value, e.g. 1. Then, for every remaining MDC pointcandidate of the list, a maximum load value is obtained by calculatingthe maximum of the real time loads in both directions for all redundanttransmission paths between the serving and drift base stations and thecandidate and for the single transmission path between the candidate andthe RNGW A. If the obtained maximum real time load value of thecandidate is smaller or equal than the set minimum load value, theminimum load value is set to the obtained maximum real time load valueof the candidate. If not, the candidate is dropped from the MDC pointcandidate list.

In step 4 of FIG. 2, the fourth criterion with the lowest priority isfinally applied by minimizing the MDC processing load for all remainingMDC point candidates.

FIG. 6 shows an example of an algorithm, which can be used in step 4.Initially, a minimum MDC load is set to a predetermined value, e.g. 1.Then, for every remaining candidate in the MDC point candidate list, itis checked whether the MDC load of the candidate is smaller than the setminimum MDC load. If so, the minimum MDC load is set to the MDC load ofthe candidate. If not, the candidate is dropped from the MDC pointcandidate list.

If the result of step 4 in FIG. 2 indicates a single MDC point, thissingle MDC point is output as the best or appropriate MDC point. If not,a single MDC point may be randomly selected among the final remainingcandidates on the MDC point candidate list, and output as the best orappropriate MDC point.

Thus, a goal based selection method for obtaining a single appropriateMDC point is provided which can be used at any initialization of a newcall.

In case of any missing information, e.g. link loads or the like, thecorresponding selection step requiring the missing information can bebypassed. If topology information is missing at the network node wherethe MDC point selection is performed, the serving base station can beused as the MDC point. However, any other selection is possible, ofcourse.

The proposed MDC point selection method may as well be applied orinitiated in cases where the network topology or the set of serving anddrift base station has changed.

FIG. 7 shows a table indicating an example of measured real time trafficloads (percentage of link capacity) towards neighbor nodes, wherein amaximum or real time traffic load may be set to 80% (in this example,all traffic is real time traffic). Furthermore, the table in FIG. 7shows respective MDC loads of each network nodes shown in thearchitecture of FIG. 1. In particular, the table of FIG. 7 presentsmeasurement information to be used in addition to the network topologyinformation, which each base station B to J has available for applyingthe selection criteria for selecting the MDC point.

In the following, the above described selection method is applied usingthe topology of FIG. 1 and the measurement results of FIG. 7.

In the first criterion according to FIG. 3, the minimum value of themaximum MDC leg length will finally be set to three hops. Thus the nodesB and F will be the remaining MDC point candidates in the candidatelist. Only these two network nodes satisfy the criteria that the maximumlength of the MDC legs is not higher than the minimum value, i.e. threehops.

Then, the second criterion according to the algorithm in FIG. 4 leads toeight hops as the minimum value of the total hop count. Still, bothnetwork nodes B and F satisfy this second criterion and thus remain onthe candidate list. Accordingly, the result of the topology-based firstand second selection criteria leads to a candidate list comprising thenetwork nodes B and F.

As regards the third criterion according to FIG. 5, the maximum realtime load is 60% obtained on the link from node B to node C. However,this link is comprised in the transmission paths of both MDC pointcandidates. Thus, still both candidates remain on the candidate list.

According to the final fourth criterion, the individual MDC processingloads of the remaining MDC point candidates are compared, wherein theMDC processing load of the candidate node F (55%) is substantially lowerthan the MDC processing load of the candidate node B (70%). Thus, step 4in FIG. 2 leads to a single MDC point, i.e. network node F, which willbe output as the best MDC point. This result corresponds to thesituation shown in FIG. 1.

Due to the fact that the suggested selection method leads to anoptimized MDC point with minimized load values and link lengths, lowerdelays for MDC traffic and more efficient network utilization can beachieved. Furthermore, the method is simple enough to be implementedwithin the base stations. However, in case the required scalabilityleads to a problem in bigger RAN topologies, multicast-like transmissionshould be provided. Additionally, a scheme suitable to distribute themeasurement results either through a centralized resource or in adistributed manner among the network nodes is needed if e.g. trafficloads or MDC processing loads are used in the MDC point selectionprocess.

However, the method and system for finding an optimized MDC pointlocation, as proposed above, might lead to an excessive amount of MDCrelocations, which is not desirable. Moreover, heavy calculations arerequired in connection with each MDC relocation. To alleviate thisproblem, a validity checking functionality can be introduced to reducethe number of MDC relocations in the RAN. According to the validitychecking functionality, the MDC point or functionality will not be movedor relocated to a possibly better location, if the current MDC point isstill valid, i.e., if the initial or slightly stricter constraints arestill met. For example, the set maximum load threshold could be slightlyreduced.

Thereby, the amount of MDC point calculations and relocations can bereduced to those cases where the location of the current MDC point nolonger meets the preset constraints.

The validity checking functionality can be activated each time there isa change in the active set of base stations. When the MDC pointcalculation is triggered, the MDC point validity checking functionalitywill first check whether or not the current MDC point meets theconstraints. If the answer is yes, no further calculations are done andthe MDC functionality is not relocated—even though the active set ofbase stations would need to be updated. In logical terms, the validitychecking functionality can be expressed as indicated in FIG. 8. It isnoted that the way how the constraints (initial or stricter) are checkeddepends on the used MDC point selection method.

FIG. 9 shows a corresponding enhancement of the flow diagram of FIG. 2,to incorporate the above validity checking functionality. In particular,the validity checking functionality is incorporated in step 0 and thesubsequent conditional branch operation, which are only performed if theset of base stations, e.g. IP BTSs, has changed.

FIG. 10 shows a specific example of the logical expression of FIG. 8,wherein a slightly stricter constraint, i.e. 90% of load threshold, isapplied to the current MDC point. Of course, any other percentage can beapplied to implement a lower threshold and thus a stricter constraint.Furthermore, stricter threshold values may as well be applied to one orall other MDC point selection constraints.

In the example of FIG. 7, it is now assumed that node G corresponds tothe current MDC point which had been selected previously. With the aboveadditional validity checking functionality, this current or old MDCpoint (node G) would be selected again or maintained, instead of node F,in the example of FIG. 7, because the constraints were still met.However, in this case, only a simple check of the load value in the MDCload column of the table of FIG. 7 was required. Hence, in addition todramatically reducing the number of MDC relocations, the proposedvalidity checking functionality will considerably reduce the amount ofMDC point selection calculations done at the base stations.

Furthermore, in the above preferred embodiment, a fallback scheme of MDCselection on topology information inconsistency (MSTII) may be providedfor the following reasons.

Upon selection of an MDC point for a call, the correct RAN topologyinformation is needed. Due to a component congestion or failure, anynetwork component such as link or node might stop its serving functionin the RAN and cause a change of RAN topology. The change of the RANtopology will then trigger an update of RAN topology information kept ineach node in RAN. The update usually needs a few seconds, called asconverging period, to accomplish. During the converging period, the RANtopology information in different nodes is different—this is called astopology information inconsistency. If MDC point selection for a call isdone with inconsistent or wrong RAN topology information, its associatedMDC relocation will be a wrong MDC relocation and its associated legaddition would be a wrong leg addition. The incorrect MDC relocation andleg addition will add high but meaningless processing and transportationcost to RAN and additionally cause some irregular RAN transportationproblems.

The MSTII fallback scheme is therefore adapted to identify the topologyinformation inconsistency problem during its happening and to preventthe wrong MDC relocations and wrong leg additions during the convergingperiod. In particular, the MSTII fallback scheme consists of twosub-schemes, a first sub-scheme for detecting the topology informationinconsistency and a second topology inconsistency fallback sub-scheme.The first sub-scheme discovers and indicates the beginning, continuing,and the ending of topology information inconsistency, to the secondsub-scheme. The second sub-scheme acts to prevent the wrong MDCrelocations and wrong leg additions during the converging period,according to the indications from the first sub-scheme. The first andsecond sub-schemes may be implemented at an MDC module provided at e.g.a centralized resource managing device or at an individual network nodesuch as an IP BTS. In the present context, the MDC module corresponds toan abstraction of one or more functions that complete the task or tasksrelated to at least MDC point selection. The MDC module may however alsocover the tasks of relocation triggering and/or management of the activeBTS set provided for soft handover of a call. This means that the MDCmodule may recommend to add a BTS into the active BTS set due to aradio-leg-addition request, or recommend to drop a BTS from the activeset due to no enough network resources for it.

FIGS. 11 and 12 show explanatory algorithms for the above first andsecond sub-scheme, respectively, which may be implemented as hardwareunits or subroutines continuously running at or for the MDC modulebefore, during and after a topology information inconsistency. The firstexplanatory sub-scheme of FIG. 11 for detecting a topology informationinconsistency may be adapted to work with a linkstate routing protocol,e.g. OSPF, or a flooding scheme. If there is an updating of the RANtopology, e.g. a start of flooding or a receipt of an updating messagefrom another node, an actual topology changing process(“Topology-changing”) is indicated or signaled to the MDC moduleprovided at the own node and a timer functionality provided at the MDCmodule is started with a predetermined value, e.g. 1.5 s, which may beestimated based on the specific RAN topology. Then, when the timer hasexpired, a changed topology (“Topology-changed”) is indicated orsignaled to the MDC module provided at the own node. The correspondingreaction at the MDC module is defined by the explanatory secondsub-scheme shown in FIG. 12. If a topology changing process is indicatedto the MDC module, it keeps rejecting any leg-addition request andsuspends MDC point selection for any call or connection. Then, if achanged topology is indicated to the MDC module, it may optionallyupdate the set of MDC-capable nodes according to a topology aggregationscheme (TAS), it then returns to normal functioning and resume its MDCpoint selection function. Thus, wrong leg additions during a topologyconverging period can be prevented.

The TAS is provided to automatically set the MDC-capable nodes, whichmay be only a small subset of all nodes in the RAN, for all possible MDCrelocations and thus enables a substantial reduction of the associatedexecution times of MDC relocation procedures for the calls in the RAN,while keeping the benefit of MDC. The MDC-capable nodes areautomatically picked up, according to the network topology informationavailable from the routing table, e.g. OSPF, or obtained from aninformation exchange among the nodes in the RAN. Whenever the RANtopology changes, this scheme will update the subset of MDC-capablenodes according to the new topology. Other nodes, called as leaf nodeswhich are not in the current MDC-capable subset of the RAN, cannot serveas MDC points for calls.

As an estimation, the TAS may enable a reduction in MDC costs by morethan ⅔ and even more than 95%, while maintaining the benefit of MDC.

In the following TAS examples are described based on topology examplesshown in FIGS. 13 and 14. The RAN can be described as an undirectedgraph G(N, L), where N denotes the set of all nodes or BTSs, e.g. IProuters, in the RAN and N={N1, N2, . . . Nk}, and L denotes the set ofall links of the graph G(N, L). Then, M which is a subset of N, denotesthe MDC-capable set of nodes of the RAN. Whenever the RAN topologychanges, triggered e.g. by a routing table update, in a node Nicontained in N, M is reset to an empty set. Then, each node Nj having atleast two links which connect to other RAN nodes is added to the set Mof MDC-capable nodes.

Whenever a call is initiated or a leg is added to or removed from anexisting call, the immediate node connected by the initial call (e.g.,Node E6 for the call in FIG. 14 when event2 has not happened yet) at thetime of call initiation or, the current MDC point (e.g., Node E5 for thecall in FIG. 14) when the call is already initiated, does the following:(1) Call an MDC point selection function (algorithm) to find thesuitable MDC point for the call from Set M and (2) Trigger MDCrelocation procedure (if the call exists and MDC relocation needed) ortrigger MDC installation procedure (if the call is in initiation).

In FIG. 13, a possible topology of an IP RAN is shown under a currentconsideration for the IP RAN. In the example of FIG. 13, the MDC-capablesubset M=(A, B, C, D, E, F, G, H, I, J, C2, E2). When a call isinitiated or a leg is added to or removed from an existing call, thesuitable MDC point is one of the nodes in M. For the call shown in FIG.13, the MDC point is located at node E. The number of possiblerelocations for a given call are calculated as the number of allpossible combinations between a current MDC point and its next new MDCpoint. For example, in the network topology of FIG. 13, there areprovided 12 MDC-capable nodes and 37 nodes in all. The number ofpossible relocations is thus 12×11/2 when using TAG and 37×36/2 when notusing any topology aggregation. Hence, the number of possible MDCrelocations under the above TAS example is 66, i.e. 12×11/2, while thenumber of possible MDC relocations without TAS would amount to 666, i.e.37×36/2. Thus, the TAS example leads to a 90% reduction of possible MDCrelocations.

FIG. 14 shows a second topology example which may correspond to a futureIP RAN network. Again, the MDC-capable subset M=(A, B, C, D, E, F, G, H,I, J, D2, E5, E6). When a call is initiated or a leg is added to orremoved from an existing call, the suitable MDC point is one of thenodes in M. For example, the call shown in FIG. 14, when event 2 hashappened, the MDC point is relocated to node E5 from node E6. However,the number of possible MDC relocations under the above TAS example is78, i.e. 13×12/2, while the number of possible MDC relocations withoutTAS would amount to 946, i.e. 44×43/2. Thus, the TAS example leads to a91.7% reduction of possible MDC relocations.

It is noted that the present invention is not restricted to the abovepreferred embodiments, but can be used in any network environment wherea plurality of redundant transmission paths are combined at a combiningpoint to a single transmission path. Furthermore, the method is notrestricted to the specific selection criteria indicated in the abovesteps 1 to 4. Any selection criterion suitable for obtaining anappropriate combining point can be used in the priority based selectionmethod. Moreover, the allocation of the priorities may be changed in anymanner suitable to obtain a combining point appropriate to a particularapplication. Thus, the preferred embodiments may vary within the scopeof the attached claims.

1. A method of selecting a combining point at which at least tworedundant transmission paths are combined to a single transmission pathin a transmission network comprising at least two selectable combiningpoints (B-J), said method comprising the steps of a) using at least twomeasurement-based selection criteria for selecting said combining point;b) allocating different priorities to said at least two selectioncriteria; and c) using the selection result of a selection criterionwith a higher priority as a constraint for a selection based on aselection criterion with a lower priority.
 2. A method according toclaim 1, wherein said at least two selection criteria comprise aselection criterion applied to measured lengths or loads of said atleast two redundant transmission paths and/or said single transmissionpath.
 3. A method according to claim 1, wherein said at least twoselection criteria comprise a selection criterion applied to measuredprocessing loads of said selectable combining points.
 4. A methodaccording to claim 2, wherein said at least two selection criteriacomprise a first criterion of minimizing the maximum length of said atleast two redundant transmission paths, a second criterion of minimizingthe maximum total length of said at least two redundant transmissionpaths and said single transmission paths, a third criterion ofminimizing the maximum traffic load on said at least two redundanttransmission paths and said single transmission path, and a fourthcriterion of minimizing the processing load of said combining point. 5.A method according to claim 4, wherein said maximum length and maximumtotal length are determined by counting hops of said single andredundant transmission paths, respectively.
 6. A method according toclaim 4, further comprising the step of allocating the highest priorityto said first criterion, the second highest priority to said secondcriterion, the third highest priority to said third criterion, and thelowest priority to said fourth criterion.
 7. A method according to claim4, wherein said third criterion is applied by monitoring and updatingreal time traffic loads using an averaging function.
 8. A methodaccording to claim 7, wherein said averaging function is an exponentialaveraging function.
 9. A method according to claim 1, further comprisingthe step of transmitting load measurement results or load reportsmutually between said at least two selectable combining points (B-J) atpredetermined intervals.
 10. A method according to claim 1, furthercomprising the step of transmitting load measurement results from saidat least two selectable combining points (B-J) to a centralizedresource, which will distribute the load information to all possiblecombining points (B-J) at predetermined intervals.
 11. A methodaccording to claim 1, further comprising the step of transmitting loadreports from said at least two selectable combining points (B-J)directly to all other possible combining points (B-J) at predeterminedintervals without any intervention of a centralized resource.
 12. Amethod according to claim 1, further comprising the step of setting amaximum load threshold to be considered during said selection of saidcombining point.
 13. A method according to claim 12, wherein saidmaximum load threshold defines the maximum allowable real time load onused links of said at least two redundant transmission paths and/or themaximum allowable class x load on used links of said single transmissionpath.
 14. A method according to claim 12, wherein said maximum loadthreshold defines the maximum allowable processing load in said selectedcombining point.
 15. A method according to claim 1, further comprisingthe step of bypassing a selection criterion if required measurementvalues are not available.
 16. A method according to claim 1, furthercomprising the step of dropping a redundant transmission path from saidat least two redundant transmission paths, if said method does not leadto a selection of a combining point.
 17. A method according to claim 16,further comprising the step of rejecting a corresponding new call ifonly one redundant transmission path is left and said method does notlead to a selection of a combining point.
 18. A method according toclaim 1, wherein said selection method is used after a change in thenetwork topology.
 19. A method according to claim 1, wherein apreviously selected combining point is maintained if said currentcombining point at least still meets said at least two measurement-basedselection criteria.
 20. A method according to claim 19, wherein at leastone stricter selection criterion is applied to said previously selectedcombining point.
 21. A method according to claim 20, wherein said atleast one stricter selection criterion corresponds to 90% of a loadthreshold value applied to said previously selected combining point. 22.A method according to claim 1, further comprising the steps of detectinga topology inconsistency, and preventing relocations of said combiningpoint during said detected topology inconsistency.
 23. A methodaccording to claim 22, further comprising the steps of starting a timerfunction in response to said detection step, and allowing relocationsafter the expiry of said timer function.
 24. A method according to claim1, wherein a subset of nodes capable of being selected as the combiningpoint in said selection step a) is determined based on the topology ofsaid transmission network.
 25. A method according to claim 24, whereinsaid determination is repeated after a change in the network topology.26. A method according to claim 24, wherein said subset of capable nodesis selected based on their number of links connecting to other nodes.27. A network node for selecting a combining point at which at least tworedundant transmission paths are combined to a single transmission pathin a transmission network comprising at least two selectable combiningpoints (B-J), said network node being arranged to use at least twomeasurement-based selection criteria with different priorities forselecting said combining point, and to use the selection result of ahigher priority selection criterion as a constraint for a selectionbased on a lower priority selection criterion.
 28. A network nodeaccording to claim 27, wherein said combining point is a macro diversitycombining point in a radio access network providing access to anIP-based network.
 29. A network node according to claim 28, wherein saidselectable combining points are base station devices (B-J).
 30. Anetwork node according to claim 27, wherein said network node is a basestation device (B-J).
 31. A network node according to claim 27, whereinsaid network node is a centralized resource managing device.
 32. Anetwork node according to claim 27, further comprising means fordetecting a topology information inconsistency, and means for preventingrelocations of said combining point during said topology informationinconsistency.
 33. A network node according to claim 32, wherein saiddetecting means comprises a timer functionality.